72 HOLOTHURIAN PLATES. 



eighty-eighth part of an inch in diameter, and are roughly octa- 

 gonal in outline. The upper surface is smooth, convex, and 

 perforated with twelve small holes, four of these holes being near 

 the centre, and eight placed at regular distances apart, just inside 

 the margin (Fig. lo). The under-side is concave and smooth, 

 with central boss, as shown in section (Fig. ii) ; and it has only 

 nine perforations (Fig. 12), the reason for this being that the four 

 central perforations of the upper surface join together (Fig. 11) 

 before reaching the under side, and are there represented by one 

 perforation only, placed in the middle of the central boss. These 

 organisms are apparently composed of carbonate of lime, as 

 they effervesce on the api)lication of an acid. They still possess 

 a dull whitish, pearly lustre. I searched the shale diligently, to 

 see if I could meet with any anchor-shaped " feet," such as we 

 find in connection with the plates of the recent Synapta, but 

 nothing of that kind turned up. I have examined a long list of 

 Scottish Carboniferous Shales, without finding any other speci- 

 mens of these minute fossils, and I know of no recent organism 

 that is at all like them. May they not be the ancient prototypes 

 of the " wheels " of Myriotrochtis 1 A slide of unnamed Holo- 

 thurian Plates from Corsica, in my possession, shews 

 three kinds of oval plates, perforated by four, six. Fig. 13. 

 and eight ovoid holes respectively; see Fig. 13. 

 They are all very much smaller than the Carbon- 

 iferous species. 



In examining the Orchard Shale, I came across 

 about a dozen of these fossils all massed together, 

 which indicates clearly that to whatever organism 

 they belonged they must have existed in considerable 

 numbers. On first seeing them, I thought they might be 

 diatoms, but their calcareous nature excludes this idea. That they 

 had been embedded in truly marine strata, is clearly indicated by 

 the fossils which were found accompanying them : — such, ^.^., as 

 riatycr'uius, G^-iffithidcs^ Dif/irocaris, Fcuestdla^ Froductus^ 

 Spirifcni^ Liiigula, etc. I have named this Carboniferous Holo- 

 thurian Tiochopahvus (ancient wheel) Youngiafius, after Mr. John 

 Young, of the Hunterian Museum, Glasgow, who, I believe, was 

 the first to find it. 



